There are purposes for rotary tools, but you haven't listed any good ones in your question. A hacksaw or pipe saw is what you use for PVC pipe, for example - a rotary tool just isn't suited for that task. Too, an XActo saw (the manual razor saw) is a beautiful tool for cutting smaller pieces of wood, while a larger machine is called for if you need to cut larger pieces of wood. Rotary tools nearly always run far too fast for cutting any type of thermoplastic - the plastic melts instead of cutting.
Rotary tools do very well at small-scale carving, though - I know an artist who uses small rotary tools to do magnificent work in fine detail. They work well for deburring small parts, grinding the heads off mangled screws, cutting hypodermic tubing, and polishing smallish surfaces. They can be used as a router for miniature furniture, with appropriate jigging. That's mostly the limit of their usefulness, though.
If you still believe you want a rotary tool because you have some work to do that fits within those bounds... you could get by with a Dremel tool provided you really baby it; they burn out very easily. If you want more power or don't want to baby the tool so much, I'll unabashedly recommend a Black & Decker RTX - nearly twice the power as a Dremel, and while I've burned up several Dremels, I have yet to make my RTX hiccup. If you want a more industrial tool and can pay for it, move the motor outside the handpiece and go with a Foredom or Foredom-style tool. If you need to go even more industrial, look to pneumatic high-speed tools - they require a healthy air compressor, though.
